How to use experience for heroes?

I would recommend reviewing your hero skills before adding experience (exp). Select blue/purple/orange heroes. Don't waste your experiences on the green heroes.

Select the hero, then select level in the left hand and add the experience. The amount used is based on your need to help gain/level up the hero in the tasks they perform (combat/development and production). For example, the Resistance exp helps the bank-money, heroes gain exp, etc. And the Survivor (hero) provides fuel and other perks to this hero and others, etc.

in case anyone else is wondering too: tap on the hero-menu anytime (the middle one at the bottom-right). from there you can select any hero you want; when done, tap the level-button at the top-right. from there you can add your earned experience-perks (it took me a while to figure this out).